A third generation (3G) Universal Terrestrial Radio Access Network (UTRAN) comprises several radio network controllers (RNCs), each of which is coupled to one or more Node Bs. Each Node B comprises one or more base stations servicing one or more cells. The Node Bs, in turn, communicate with one or more User Equipment (UEs).
A 3G system, which includes both Frequency Division Duplex (FDD) and Time Division Duplex (TDD) modes, typically uses the RNC to distribute, (i.e., buffer and schedule), data transmissions to the UE. However, for the high speed channels of 3G cellular systems, data is distributed by the Node B. One of these high speed channels, for example, is the High Speed Downlink Shared Channel (HS-DSCH). Since data is distributed by the Node B, it is necessary to buffer data in the Node B prior to transmission to the UE.
There are many scenarios where the data that is buffered in the Node B is no longer useful, and its presence there could impede efficient operation of the system. For example, a first scenario is when a mobile UE travels from one cell to another. This will result in either an HS-DSCH cell change, whereby the UE is either serviced by another Node B, or switching between cells in the same Node B. The “old data”, (i.e., the data that is buffered within the Node B for transmission to the UE prior to the HS-DSCH cell change), is no longer useful after the HS-DSCH cell change. If the Node B continues to buffer and transmit this data, it wastes both buffering resources and radio link resources. It is desirable to delete this old data from the buffer and to cease the transmission of this data since it will save both buffering resources and radio link resources.
A second scenario relates to the radio link control (RLC) layer. The RLC layer is a peer entity in both the serving radio network controller (SRNC) and the UE. There are occasions when the RLC peer to peer protocol fails, and the RLC resets itself. The reasons for RLC failure are varied and such reasons are outside the scope of the present invention. However, once the RLC resets itself, the data previously buffered in the Node B is no longer useful since the RLC resynchronizes and restarts transmissions. This buffered data can only cause transmission delays and unnecessary use of radio resources. If transmitted, this data will just be discarded by the RLC peer entity.
A third scenario relates to the in-sequence delivery of data by the RLC in Acknowledged Mode (AM). A requirement for the AM RLC is to make sure that in-sequence delivery of protocol data units (PDUs) occurs. The RLC uses a Sequence Number (SN) associated with each PDU to ensure in-sequence delivery of PDUs to higher layers. When there is an out-of-sequence delivery, (i.e., when a PDU is missed), the RLC in the UE sends a Status Report PDU to its peer entity in the Node B, requesting retransmission of the missed PDUs. Upon receiving the Status Report PDU, the peer entity in the RNC retransmits a duplicate of the missed PDU.
It is highly desirable for the retransmitted PDUs to arrive at the RLC of the receiving side (i.e., the UE) as soon as possible for several reasons. First, the missed PDU will prevent subsequent PDUs from being forwarded to higher layers, due to the requirement of in-sequence delivery. Second, the buffer of the UE needs to be sized large enough to accommodate the latency of retransmissions while still maintaining effective data rates. The longer the latency is, the larger the UE buffer size has to be to allow for the UE to buffer both the PDUs that are held up and continuous data receptions until the correct sequence PDU may be forwarded to higher layers. The larger buffer size results in increased hardware costs for UEs. This is very undesirable.
FIG. 1 is a prior art system including an RNC, a Node B, a UE and their associated buffers. In this prior art system, a PDU with SN=3 is not received successfully by the UE. Therefore, the RLC in the UE requests its peer RLC layer in the RNC for a retransmission. Meanwhile, the PDUs with SNs=6-9 are buffered in the Node B, and PDUs with SNs=4 and 5 are buffered in the UE. It should be noted that although FIG. 1 shows only several PDUs being buffered, in reality many more PDUs (such as 100 or more) and PDUs from other RLC entities may be buffered.
As shown in FIG. 2, the retransmission of the PDU with SN=3 must wait at the end of the queue in the Node B buffer, and will be transmitted only after the PDUs with SNs=6-9 are transmitted. The PDUs in the UE cannot be forwarded to the upper layers until all PDUs are received in sequence. In this case, the PDU with SN=3 stalls the forwarding of subsequent PDUs to higher layers, (i.e. SNs=4-9), assuming all the PDUs are transmitted successfully. Note that this example only reflects 10 PDUs, whereas in normal operation hundreds of PDUs may be scheduled in advance of retransmitted data PDUs, which further aggravates transmission latency and data buffering issues.
The above scenarios are just a few of the many examples wherein the purging of data in the Node B would result in much more efficient operation of a wireless communication system.
It would be desirable to have a system and method whereby the RNC can control the purging of data buffered in the Node B that is no longer useful. Under many circumstances, deletion of this buffered data would result in more efficient operation of the system.